An example of an optical read/write system is shown in FIG. 1. It can be seen that a beam is created and emitted by a laser diode 1. The beam first encounters a beam splitter 3, which splits the beam into multiple beams and reflects these beams toward a collimator 5. The collimator 5 is a type of lens that is constructed and arranged to form a collimated beam. After the split beams pass through the collimator 5, they are parallel with each other and until they pass through an objective lens 7. The objective lens 7 bends the beams so that they all converge at a focal point, which is preferably at the surface or information layer of the media 9. The objective lens 7 may be mounted on a lens actuator that can adjust the distance between the lens and the media, which thereby controls focusing and the radial position of the focused beam.
Once the light has been properly focused on the information layer 9, data marks 10 on the storage media will reflect back less light than the regions between them. The reflected light will then pass back through the objective lens 7, the collimator 5, and the beam splitter 3. Next, the reflected beams pass through a wedge prism 11 that focuses the beams on a photodiode detector 13. The detector 13 measures the signal corresponding to data 10 on the media 9 and may also detect errors in the focusing and positioning of the objective lens 7.
As with any data read/write system, optical systems are vulnerable to flaws and other manufacturing variations, dirt, scratches, and other factors that may result in degraded performance. The effects of some of these factors are easily minimized. For example, some media will include a protective substrate between the information layer and the laser beam. The substrate is generally provided on the outer surface of the storage media. In this case, the beam is still focused on the information layer, thereby reading information through the protective substrate. Focusing the beam at a point beyond the surface of the substrate onto the data marks minimizes the effect that undesirable elements, such as dust particles on the substrate, have on the signal read from the media.
However, other undesirable elements are prevalent and have heretofore gone uncorrected. Two particularly troublesome elements pertain to the thickness of the objective lens and the frequency at which the laser beam operates. Unwanted variations in the thickness and curvature of the objective lens manifest themselves as spherical aberrations in the laser beams passing therethrough. Spherical aberrations cause concentric rings to form around the focused spot on the media, making it difficult to read the stored information. Unwanted variations in the frequency of the laser beams also manifest themselves as spherical aberrations in the beam upon exiting the objective lens, as the lens is constructed and arranged to match the specific frequency of the laser.
Because all manufacturing processes result in components having certain tolerances and variations, spherical aberrations are difficult to avoid using an economical manufacturing process. Moreover, the relatively small radius of the curve used in objective lenses makes spherical aberrations even more difficult to avoid, control and predict. Variations in the wavelength of the laser diode light source are equally as difficult to control and predict using economically feasible manufacturing techniques.
Prior art attempts at accounting for such spherical aberrations have included placing plate glass of varying thickness in a divergent or convergent beam of light in an attempt to minimize the spherical aberration. Another attempt involved slightly de-collimating the beam entering an objective lens to generate spherical aberration of the opposite sign in an attempt to cancel out spherical aberration in the objective lens. Minimal success was met with each.
There is a need for a method of minimizing or eliminating spherical aberration caused by variations in objective lens thickness and laser frequency. Preferably, the method could be used alone or in conjunction with prior art methods.